Accuracy of two electromagnetic soil water content sensors in saline soils

Information on the accuracy of commercially available electromagnetic (EM) soil water content sensors in a variety of soil types and over a range of volumetric water content (VWC) and salinity levels is still limited. A laboratory study was conducted to evaluate the accuracy of the Acclima Digital Time Domain Transmissometry (TDT) and the Toro Turf Guard Frequency Domain Reflectometry (FDR) sensors in a loamy sand and a loam soil at soil salinity levels from 2 to 18 dS m^-1. Overall linear association between gravimetric and sensor-determined VWC across all salinity levels revealed coefficients of determination between 0.86 and 0.97. In addition to a good linear model fit, a significant quadratic association was also detectable for the TDT sensor in loamy sand at 12 dS m^-1 and in loam at 12 and 18 dS m^-1. Overall, both sensors reliably measured VWC in both soil types at all salinity levels, however data suggests that different soil types require separate calibration. Moreover, accuracy of the sensor readings can be further improved if differential calibration is applied for different salinity levels. Regression analysis indicated that with increasing salinity, the Turf Guard sensors showed an upward bias (intercept for higher salinities greater than for 3 dS m^-1), whereas the TDT sensor exhibited a downward bias (intercept smaller than for low salinitiy levels).